The following plant-derived plant growth factors are known; barley-derived, fat-soluble fatty acid having a molecular weight of 600 or less [Journal of Plant Physiology, Vol. 121, pp. 181-191, 1985], pine-derived growth factor consisting of oligosaccharides having a molecular weight of 1000 or less [Plant Cell, Tissue and Organ Culture, Vol. 26, pp. 53-59, 1991], carrot-derived, heat-stable grow th factor having a molecular weight of about 700 [Plant Science, Vol. 51, pp. 83-91, 1987], and black Mexican maize-derived growth factor which has a molecular weight of 1350 or less, has oligosaccharide-like characteristics and is not adsorbed to either of anion-exchange resin or cation-exchange resin in a buffer at pH of 5 [Journal of Plant Physiology, Vol. 132, pp. 316-321, 1988].
It is difficult to isolate and purify such known plant-derived plant growth factors, and there is no known technique for mass-production of these factors. In order to use a plant growth factor as a plant growth promoter, there is a need to find out a mass-producible plant growth factor. In order to achieve industrial production, it is need to provide a lower molecular plant growth promoter.